For example, in an X-ray radiographic apparatus or an X-ray CT apparatus used in medical or industrial settings, a scattered X-ray eliminating grid is generally disposed between a subject and an X-ray detector with the goal of preventing X-rays scattered, by the subject from impinging the X-ray detector.
This type of grid comprises numerous foil shaped. X-ray absorbing substance, which are made of lead and the like, and intermediate substance, which are interposed as spacers between the foil shaped. X-ray absorbing substance; furthermore, the X-ray absorbing substance are disposed parallel to the primary X-rays. In a regular X-ray fluoroscopic apparatus, an X-ray CT apparatus, or the like, wherein an X-ray tube serves as the radiation source, a so-called focused grid is used wherein the foil shaped X-ray absorbing substance are disposed such that their surfaces extends toward and focuses on one straight line disposed at a focal distance. In addition, in special applications, a parallel grid, wherein the foil shaped X-ray absorbing substance are disposed parallel to one another, is also used.
Aluminum, a paperlike fiber, or the like is often used as the intermediate substance, and any one of these may be adopted in practical use (e.g., refer to Iida, Noboru. “X-ray Grids Made Easy to Understand.” Japanese Journal of Radiological Technology (June 1999): 529-535).
Here, however, because intermediate substances such as aluminum, fiber, or the like absorb primary X-rays, the amount of X-rays with which the subject is exposed must be increased commensurately. To compensate for this, within the conventional art, a number of proposals have been made for grids wherein air serves as the intermediate substance (e.g., refer Japanese Unexamined Patent Application Publication No. 2002-40150).
In such proposals, a structure is adopted wherein multiple pins, which are substantially parallel to one another and spaced apart by a spacing, are arrayed at portions opposing a frame that forms the outer frame of a grid, and tape, which serves as an X-ray absorbing substance, is successively applied, to and wound around, each pin. In these proposals, the tape that serves as the X-ray absorbing substance is made of polyethylene terephthalate resin and is coated with tungsten powder.
In addition, already under consideration is an even more practical proposal, namely, a scattered X-ray eliminating grid wherein; guide slit plates are disposed such that they are fixed relative to one another, namely, parallel to and spaced apart from one another by a prescribed distance; in each guide slit plate, numerous guide slits are formed and numerous metal foils, which serve as the X-ray absorbing substance provided between the guide slit plates, are fitted in the numerous guide slits such that they are parallel to the primary X-rays; and both ends of the metal foils are inserted into opposing guide slits of each of the guide slit plates such that the metal foils are held on the outer sides of the slits and an urging means applies tension to one end or both ends of the metal foils. In this proposal, a tension rod and a fixed rod, which is sheathed in an elastic tube (e.g., a silicon tube), are inserted and pulled through holes in each of the metal foils; thereby, any variation in the hole-to-hole spacing of the metal foils is absorbed, and tension is exerted evenly over all of the metal foils.
Nevertheless, in the urging means of the proposal discussed above, while the diameter of the hole in each of the metal foils is limited, the diameter of the fixed rod must be large enough to ensure its rigidity; as a result, the outer diameter and the inner diameter of the elastic tube is minimized, and the thickness of the cross section of the elastic tube is extremely small; namely, the spring constant k, which is determined when the elastic tube is compressed, is large. Accordingly, if there is any variation in the hole-to-hole spacing of the metal foils, then when the rods are pulled the amount of compression x of the elastic tube will be large when the hole-to-hole spacing of the metal foils is short and small when the hole-to-hole spacing of the metal foils is long. Namely, because the tension is expressed by the equation F=kx, tension cannot be exerted, evenly over all of the metal foils if the spring constant k is large and there is variation in the hole-to-hole spacing of the metal foils, which is a problem. Consequently, the scattered X-ray eliminating grid is no longer functional because, even in the tensioned state, some of the metal foils extend perfectly straight while some are deformed, namely, curved.
The thickness of the numerous metal foils that serve as the X-ray absorbing substance is usually small, namely, several tens of microns; furthermore, if tungsten, a tungsten alloy, molybdenum, or the like is selected as the material of the metal foils, then the strength of the metal foils will be relatively high, as will the limit point at which the holes of the metal foils deform, even if the rods are inserted and pulled through the holes of each of the metal foils, which therefore is not a problem. However, if a comparatively weak material—such as lead, a lead alloy, copper, a copper alloy, iron, a ferrous alloy, or nickel—or a material of a similar strength is selected as the material of the metal foils, then the limit point at which the holes of the metal foils will deform when the rods are inserted and pulled therethrough will be low and differentially large tensions will be exerted if the hole-to-hole spacing of the metal foils is short; thereby, the holes will deform, it will no longer be possible to apply tension properly and, in the worst case, the metal foils will sever.
The present invention was conceived considering such circumstances, and it is an object of the present invention to provide a method of inexpensively and stably manufacturing a scattered X-ray eliminating grid wherein air serves as an intermediate substance even as metal foils, which serve as X-ray absorbing substance, can be accurately positioned and held.